This invention relates to synthetic vascular grafts, and more particularly to synthetic woven vascular grafts including a stiffening component to provide a non-crimped self-supporting graft.
vascular grafts of synthetic materials are widely used for the replacement of segments of human blood vessels. Synthetic vascular grafts have taken a wide variety of configurations and are formed of a wide variety of materials. Among the accepted and successful vascular graft implants are those formed from a biologically compatible material in tubular form which retain an open lumen to permit blood to flow normally through the graft after implantation. The biologically compatible materials include thermoplastic materials such as polyester, polytetrafluoroethylene (PTFE), silicone and polyurethanes. The most widely used are polyester fibers and PTFE. The polyester fibers, usually Dacron, may be knit or woven and may be of a monofilament, multifilament or staple yarn. or combination of each.
There are a wide variety of synthetic vascular grafts presently in use. An important factor in the selection of a particular graft is the porosity of the substrate of which the graft is formed the resiliency of the tubular construction and the strength requirements for the implant. Porosity is significant, because it controls the tendency to hemorrhage during and after implantation and influences ingrowth of tissue into the wall of the graft.
Synthetic fabric vascular grafts may be of a woven, knitted with or without a velour construction. A synthetic vascular graft having a warp-knit construction is disclosed by William J. Liebig in U.S. Pat. No. 3,945,052. Another graft having a warp knit double-velour construction is described by Liebig and German Rodriquez in U.S. Pat. No. 4,047,252. William J. Liebig and Dennis Cummings describe a synthetic woven double-velour graft in U.S. Pat. No. 4,517,687; the velour loops being formed of warp yarns which are preshrunk multifilament yarns. These three issued United States patents for synthetic vascular grafts are assigned to the assignee of this application.
U.S. Pat. No. 4,892,539 issued to Durmus Koch describes a synthetic fabric woven graft with a single velour on the outer surface. The graft is described as woven from multifilament polyester yarns, specifically described as texturized, with the single outer velour formed of filling yarns with each velour loop extending outside a plurality of warp yarns.
After knitting or weaving multifilament yarns into tubular forms, the grafts are compacted by a method such as disclosed in U.S. Pat. Nos. 3,853,462 to Ray E. Smith and No. 3,986,828 to Harmon Hoffman and Jacob Tolsma also assigned to the same assignee as this application. Compaction results in shrinking of the yarns and generally reduces the overall porosity of the fabric substrate. These tubular grafts after compacting have a diameter from about 6 mm to 40 mm.
Subsequent to compacting, synthetic tubular fabric grafts are crimped. Crimping involves forming ridges in the wall of the grafts to eliminate the danger of kinking or collapse of the tubing when flexed and results in uniform, regular, circular corrugations which maintains uniform strength over the entire surface of the graft tubing. This applies to both the woven and knit fabric vascular grafts. Examples are shown by L. R. Sauvage in U.S. Pat. No. 3,878,565 who describes a tubular textile synthetic fiber prosthesis of a body having a multiplicity of outwardly extending fiber loops. In FIG. 2a, the graft body is crimped into irregular, circumferential corrugations. The degree of protection afforded by irregular corrugation varies over the lengths of the tube and can fall below the required level of protection at specific regions. The warp-knit and woven grafts described above in U.S. Pat. Nos. 3,945,052, 4,047,252 and 4,517,687 are circularly crimped. The graft in U.S. Pat. No. 4,892,539 is crimped in a spiral fashion. Crimped or corrugated walls can disrupt blood flow and create areas of thick tissue buildup, due to the profile.
S. Polansky in U.S. Pat. No. 3,304,557 avoids crimping in vascular prothesis by forming a tube with repeating reinforcing ring sections. These reinforcing ring sections incorporate reinforcing picks adjacent only the outer surface. He proposes that the annular rings can be in the form of a helix, alternating rings and helix-loops. These latter suggestions are similar to the tubular prosthesis of I. B. Medell in U.S. Pat. No. 3,479,670 wherein an open mesh tube is wrapped with two polypropylene monofilament right-hand and left-hand helices and fused to penetrate partially the exterior of the tube. In U.S. Pat. No. 3,272,204 to C. Artandi and L. D. Bechtol sew a Dacron fabric to Teflon rings or a helix to prevent an absorbable collagen reinforced graft tube from collapsing.
Selection of a particular type of graft substrate by a vascular surgeon depends upon several factors. Among the factors included is the particular location of the implantation. This also dictates the size of the graft in order to maintain a sufficiently large or small lumen to accommodate normal blood flow in the region of implantation. The ultimate strength requirements and blood pressure in the location of implantation also affect the selection. Generally, the woven grafts provide greater strength and reduced porosity, but are generally considered to be more difficult to handle and suture. Velours are often preferred because the velour surfaces facilitate growth of tissue into the loops extending from the surface of the velour fabric. The knitted grafts are generally softer and more easily sutured, but are generally more porous. Depending on the location of the implant and heparinization condition of the patient, synthetic fabric grafts generally must be preclotted with the patients blood before implantation. Preclotting may not be essential with a woven graft, but is generally recommended nonetheless.
Tubular grafts of smaller diameter, for example, 6 mm and below are often utilized in peripheral regions of the body and appendages. Today, the most successful in this respect are grafts of PTFE of the material disclosed by Robert W. Gore in U.S. Pat. Nos. 4,187,390 and 3,953,566. These grafts are formed by extrusion of the PTFE material. While accepted for use in small diameter applications, PTFE grafts often require surgical replacement within relatively short periods of time compared to the larger diameter fabric vascular grafts described above.
Accordingly, it is desirable to provide a synthetic fabric vascular graft suitable for a wide variety of dimensions and diameter providing the benefits of woven grafts, but which do not require crimping and will be self supporting and maintain an open lumen.